Under the Transmission Control Protocol/Internet Protocol (TCP/IP), when a client provides a symbolic name (a Uniform Resource Locator or URL) to request access to an application program or another type of resource, the host name portion of the URL needs to be resolved into an IP address of a server for that application program or resource. For example, the URL (e.g., http://www.foundrynet.com/index.htm) includes a host name portion www.foundrynet.com that needs to be resolved into an IP address. The client first provides the host name portion to a local name resolver, which then queries a local Domain Name System (DNS) server to obtain a corresponding IP address. If a corresponding IP address is not locally cached at the time of the query, or if the time-to-live (TTL) of a corresponding IP address cached locally has expired, the DNS server then acts as a resolver and dispatches a recursive query to another DNS server. This process is repeated until an authoritative DNS server for the domain (e.g., foundrynet.com, in this example) is reached. The authoritative DNS server returns one or more IP addresses, each corresponding to an address at which a server hosting the application (“host server”) under the host name can be reached. These IP addresses are propagated back via the local DNS server to the original resolver. The application at the client then uses one of the IP addresses to establish a TCP connection with the corresponding host server. Each DNS server caches the list of IP addresses received from the authoritative DNS server for responding to future queries regarding the same host name, until the TTL of the IP addresses expires.
To provide some load sharing among the host servers, global server load balancing (GSLB) switches are sometimes used as proxies for authoritative DNS servers, together with one or more site switches each associated with one or more host servers. Each site switch provides the GSLB switch with current site-specific information related to the host servers associated with the site switches. When the DNS server sends the list of IP addresses in response to a client query, the GSLB switch evaluates this list by applying a GSLB policy in conjunction with using the information provided by the site switches. The GSLB policy contains, among other possible specifications, the metrics that are to be applied to the list of IP addresses and the order in which they are to be applied to select the best IP address. After the GSLB switch evaluates the addresses returned by the DNS server using the metrics in the policy, the GSLB switch sends the ordered address list, having the optimum address for access listed at the top, to the client. An example of a GSLB system and description of associated metrics are disclosed in U.S. application Ser. No. 10/376,903, entitled “GLOBAL SERVER LOAD BALANCING,” filed Feb. 28, 2003, assigned to the same assignee as the present application, and which is incorporated herein by reference in its entirety.
A given GSLB policy may specify which of the metrics are enabled and to be used for selection, parameters for the metrics (such as tolerance, limits, etc.), and the order in which these metrics should be used to evaluate the IP addresses in the DNS reply. Also, a GSLB policy can specify other configuration information, such as returning best IP address only instead of the entire IP list and so on.
Currently, the user (such as a system administrator) can only define such a GSLB policy (or other load balancing policies) globally. This global GSLB policy is applicable to all the domains for which the GSLB switch is providing GSLB. For instance, consider the example where the GSLB switch is providing GSLB for www.foo.com and www.test.com. (Also note in this example that for the domain www.foo.com, “www” is referred to as the “host” and “foo.com” is referred to as the “zone”). The user may define a GSLB policy with the following example metrics and metric-order:
Health check
Geographic
Least-response
This global policy would apply to both www.foo.com and www.test.com.
If the user wants a round-trip time (RTT) metric to be used for selection of the best IP address for the domain www.foo.com but not for the domain www.test.com, then there is currently no provision for this capability. The reason is that if the user enabled the RTT metric, then since the policy is global and applies to all domains, this metric would get enabled for both of the above domains.